1. Field
The present disclosure generally relates to cold working of holes in metal, and deals more particularly with a method and tool for cold working holes in a stack of metal and composite layers.
2. Background
It is sometimes necessary to form fastened joints between composite and metal members. For example, in the aircraft industry, metal splice elements may be used to join together sections of a composite fuselage frame. Similarly, metal shear ties may be fastened to composite outer skins.
Potential metal fatigue around holes in fastened metal structures can be reduced by “cold working” the holes. One method of cold working involves placing a split sleeve in the hole, and drawing a mandrel through the sleeve. The use of a split sleeve allows for one-sided processing and shields the hole from frictional forces generated by the high interference of the mandrel. Drawing the mandrel through the sleeve expands and causes a radial plastic flow of material in the metal structure surrounding the hole, placing the material in tension. After the mandrel is removed, an annular zone of residual compressive stresses is present in the structure surrounding the hole that may extend up to one diameter beyond the edge of the hole.
Problems are presented however, when cold working a hole formed in a stack of metal and composite layers. Expansion of the split sleeve during the cold working process may result in undesirable deformation of the composite layers surrounding the hole, and/or produce composite residue that builds up on the split sleeve. One solution to this problem involves disassembling the stack, cold working the holes in the metal layers, and then reassembling the layers into the stack. This solution, however, is time-consuming, labor intensive and may not be practical in some applications.
Accordingly, there is a need for a method and tool for cold working holes in a composite and metal stack without disassembling the stack and which may be performed from one side of the stack.